Common Language Summary Evolution
of olfaction across the dinosaurian transition to birds.
Everyone knows birds aren’t exactly bloodhounds.
The party line has been that
the
evolution of
flight in birds favored vision and balance over olfaction, and
so the sense of smell was reduced. A new study overturns that
perception, showing that as birds evolved from theropod
dinosaurs their sense of smell was maintained and actually
increased in basal birds. The olfactory bulbs of the brain,
where information on odors is processed, were measured in 157
species of non-avian dinosaurs, fossil birds, and modern-day
birds. For many of the fossil species, CT scanning provided the
first-ever glimpse into brain and olfactory bulb structure.
Rather than diminishing, the olfactory bulbs of the earliest
birds were still basically dinosaur-sized, and even increased in
relative size among early birds, including in the earliest
members of the radiation of modern-day (neornithine) birds. As a
result, the sense of smell remained very important to early
birds, even as they were simultaneously honing other parts of
their “flight computer,” and early birds may have used odors to
a much greater extent in navigation and foraging than do most
birds today. Another key finding is the unexpected importance of
smell for non-avian theropods. Many dinosaur species that we
usually regard as having used mostly vision, such as
Velociraptor, Bambiraptor and Troodon, almost
certainly also relied on smell. So, how did this notion of birds
having a weak olfactory sense become dogma? As it turns out, the
birds with the smallest olfactory bulbs are the ones we see
every day—the perching birds (crows, finches) at our feeders and
the parrots in our birdcages. It also may be no coincidence that
these are also the cleverest birds, suggesting that enhanced
smarts may decrease the need for a powerful sniffer.

Evolution in birds of the olfactory bulb, the part of the brain
where smell information is processed, passing from a dinosaur (Bambiraptor)through early birds (Lithornis,
Presbyornis) to a modern-day bird (pigeon). Courtesy of
WitmerLab at Ohio University.

Evolution in birds of the olfactory bulb, the part of the brain
where smell information is processed, passing from a dinosaur (Bambiraptor,
top)through
early birds (Lithornis,
left;
Presbyornis,
middle right) to a modern-day bird (pigeon, bottom
right). Courtesy of WitmerLab at Ohio University.

The dinosaur Bambiraptor in a turkey vulture's colours. Bambiraptor had a keen sniffer similar to that of a modern-day turkey vulture. Courtesy of Julius Csotonyi. Restrictions: use for web-based releases only, otherwise contact artist [julius.csotonyi@gmail.com]

Graph of olfactory bulb size and body size of individual bird (black symbols) and dinosaur (green symbols) species. Similarities in olfactory bulb size and body size between species, as illustrated by the dinosaur Bambiraptor, the turkey vulture, and the albatross, suggest a comparable sense of smell. Courtesy of Darla Zelenitsky

Ohio University News Release

Birds inherited strong sense of smell from dinosaurs
Study provides new details on evolution of smell, from dinosaurs
to modern-day birds

ATHENS, Ohio (April 13, 2011) –
Birds are known more for their senses of vision and hearing than
smell, but new research suggests that millions of years ago, the
winged critters also boasted a better sense for scents.

A study published today by
scientists at the University of Calgary,
the Royal Tyrrell Museum
and the Ohio University Heritage College of Osteopathic Medicine tested
the long-standing view that during the evolution from dinosaurs
to birds, the sense of smell declined as birds developed
heightened senses of vision, hearing and balance for flight. The
team compared the olfactory bulbs in the brains of 157 species
of dinosaurs and ancient and modern-day birds.

The findings, published in the
journal Proceedings of the Royal Society B, dispute that
theory. The scientists discovered that the sense of smell
actually increased in early bird evolution, peaking millions of
years ago during a time when the ancestors of modern-day birds
competed with dinosaurs and more ancient branches of the bird
family.

“It
was previously believed that birds were so
busy developing vision, balance and coordination for flight that
their sense of smell was scaled way back,” said Darla Zelenitsky,
assistant professor of paleontology at the University of Calgary
and lead author of the research. “Surprisingly, our research
shows that the sense of smell actually improved during
dinosaur-bird evolution, like vision and balance.”

In an effort to conduct the most
detailed study to date on the evolution of sense of smell, the
research team made CT scans of dinosaurs and extinct bird skulls
to reconstruct their brains. The scientists used the scans to
determine the size of the creatures’ olfactory bulbs, a part of
the brain involved in the sense of smell. Among modern-day birds
and mammals, larger bulbs correspond to a heightened sense of
smell.

“Of course the actual brain
tissue is long gone from the fossil skulls,” said study
co-author Lawrence Witmer, Chang Professor of Paleontology at
the Ohio University Heritage College of Osteopathic Medicine, “but we can
use CT scanning to visualize the cavity that the brain once
occupied and then generate 3D computer renderings of the
olfactory bulbs and other brain parts.”

The study revealed details of how
birds inherited their sense of smell from dinosaurs.

“The oldest known bird,
Archaeopteryx, inherited its sense of smell from small
meat-eating dinosaurs about 150 million years ago,” said
François Therrien, curator of dinosaur palaeoecology at the
Royal Tyrrell Museum and co-author of the study. “Later, around
95 million years ago, the ancestor of all modern birds evolved
even better olfactory capabilities.”

How well did dinosaurs smell,
especially compared to modern animals? Although scientists
haven’t been able to make an exhaustive comparison, Witmer noted
that the ancient beasts most likely exhibited a range of
olfactory abilities. T. rex had large olfactory bulbs,
which probably aided the creature in tracking prey, finding
carcasses and possibly even territorial behavior, while a sense
of smell was probably less important to dinosaurs such as
Triceratops, he said.

The team was able to make some
direct comparisons between the ancient and modern-day animals
under study. Archaeopteryx, for example, had a sense of
smell similar to pigeons, which rely on odors for a number of
behaviors.

“Turkey vultures and albatrosses
are birds well known for their keen sense of smell, which they
use to search for food or navigate over large areas,” says
Zelenitsky. “Our discovery that small Velociraptor-like
dinosaurs, like Bambiraptor, had a sense of smell as
developed as turkey vultures and albatrosses suggests that smell
may have played an important role while these dinosaurs hunted
for food.”

If early birds had such powerful
sniffers, why do birds have a reputation for a poor sense of
smell? Witmer explained that the new study confirms that the
most common birds that humans encounter today—the backyard
perching birds such as crows and finches, as well as pet
parrots—indeed have smaller olfactory bulbs and weaker senses of
smell. It may be no coincidence that the latter are also the
cleverest birds, suggesting that their enhanced smarts may have
decreased the need for a strong sniffer, he said.

Other authors on the article
include Amanda McGee, a graduate student at the University of
Calgary, and Ryan Ridgely, a research associate in the WitmerLab
at Ohio University. The research was funded by grants to
Zelenitsky from the Natural Sciences and Engineering Research
Council of Canada and to Witmer and Ridgely from the U.S.
National Science Foundation.

This website provides supplementary information as an
adjunct to the published paper. Witmer, with the skilled
assistance of
Ryan Ridgely, is responsible for
the content of the website. Content provided here is for
educational and research purposes only, and may not be used for
any commercial purpose without the permission of
L. M. Witmer and other
relevant parties.